text: #ifndef X86_64_PDA_H
  #define X86_64_PDA_H
  
  #ifndef __ASSEMBLY__
  #include <linux/stddef.h>
  #include <linux/types.h>
  #include <linux/cache.h>
  #include <asm/page.h>
  
  /* Per processor datastructure. %gs points to it while the kernel runs */ 
  struct x8664_pda {
  	struct task_struct *pcurrent;	/* 0  Current process */
  	unsigned long data_offset;	/* 8 Per cpu data offset from linker
  					   address */
  	unsigned long kernelstack;  /* 16 top of kernel stack for current */
  	unsigned long oldrsp; 	    /* 24 user rsp for system call */
          int irqcount;		    /* 32 Irq nesting counter. Starts with -1 */
  	int cpunumber;		    /* 36 Logical CPU number */
  #ifdef CONFIG_CC_STACKPROTECTOR
  	unsigned long stack_canary;	/* 40 stack canary value */
  					/* gcc-ABI: this canary MUST be at
  					   offset 40!!! */
  #endif
  	char *irqstackptr;
  	int nodenumber;		    /* number of current node */
  	unsigned int __softirq_pending;
  	unsigned int __nmi_count;	/* number of NMI on this CPUs */
  	short mmu_state;
  	short isidle;
  	struct mm_struct *active_mm;
  	unsigned apic_timer_irqs;
  	unsigned irq0_irqs;
  	unsigned irq_resched_count;
  	unsigned irq_call_count;
  	unsigned irq_tlb_count;
  	unsigned irq_thermal_count;
  	unsigned irq_threshold_count;
  	unsigned irq_spurious_count;
  } ____cacheline_aligned_in_smp;
  
  extern struct x8664_pda *_cpu_pda[];
  extern struct x8664_pda boot_cpu_pda[];
  
  #define cpu_pda(i) (_cpu_pda[i])
  
  /* 
   * There is no fast way to get the base address of the PDA, all the accesses
   * have to mention %fs/%gs.  So it needs to be done this Torvaldian way.
   */ 
  extern void __bad_pda_field(void) __attribute__((noreturn));
  
  /*
   * proxy_pda doesn't actually exist, but tell gcc it is accessed for
   * all PDA accesses so it gets read/write dependencies right.
   */
  extern struct x8664_pda _proxy_pda;
  
  #define pda_offset(field) offsetof(struct x8664_pda, field)
  
  #define pda_to_op(op,field,val) do {		\
  	typedef typeof(_proxy_pda.field) T__;	\
  	if (0) { T__ tmp__; tmp__ = (val); }	/* type checking */ \
  	switch (sizeof(_proxy_pda.field)) {	\
  	case 2:					\
  		asm(op "w %1,%%gs:%c2" : 	\
  		    "+m" (_proxy_pda.field) :	\
  		    "ri" ((T__)val),		\
  		    "i"(pda_offset(field))); 	\
   		break;				\
  	case 4:					\
  		asm(op "l %1,%%gs:%c2" : 	\
  		    "+m" (_proxy_pda.field) :	\
  		    "ri" ((T__)val),		\
  		    "i" (pda_offset(field))); 	\
  		break;				\
  	case 8:					\
  		asm(op "q %1,%%gs:%c2": 	\
  		    "+m" (_proxy_pda.field) :	\
  		    "ri" ((T__)val),		\
  		    "i"(pda_offset(field))); 	\
  		break;				\
         default: 				\
  		__bad_pda_field();		\
         }					\
         } while (0)
  
  #define pda_from_op(op,field) ({		\
  	typeof(_proxy_pda.field) ret__;		\
  	switch (sizeof(_proxy_pda.field)) {	\
         	case 2:					\
  		asm(op "w %%gs:%c1,%0" : 	\
  		    "=r" (ret__) :		\
  		    "i" (pda_offset(field)), 	\
  		    "m" (_proxy_pda.field)); 	\
  		 break;				\
  	case 4:					\
  		asm(op "l %%gs:%c1,%0":		\
  		    "=r" (ret__):		\
  		    "i" (pda_offset(field)), 	\
  		    "m" (_proxy_pda.field)); 	\
  		 break;				\
         case 8:					\
  		asm(op "q %%gs:%c1,%0":		\
  		    "=r" (ret__) :		\
  		    "i" (pda_offset(field)), 	\
  		    "m" (_proxy_pda.field)); 	\
  		 break;				\
         default: 				\
  		__bad_pda_field();		\
         }					\
         ret__; })
  
  #define read_pda(field) pda_from_op("mov",field)
  #define write_pda(field,val) pda_to_op("mov",field,val)
  #define add_pda(field,val) pda_to_op("add",field,val)
  #define sub_pda(field,val) pda_to_op("sub",field,val)
  #define or_pda(field,val) pda_to_op("or",field,val)
  
  /* This is not atomic against other CPUs -- CPU preemption needs to be off */
  #define test_and_clear_bit_pda(bit,field) ({		\
  	int old__;						\
  	asm volatile("btr %2,%%gs:%c3\n\tsbbl %0,%0"		\
  	    : "=r" (old__), "+m" (_proxy_pda.field) 		\
  	    : "dIr" (bit), "i" (pda_offset(field)) : "memory");	\
  	old__;							\
  })
  
  #endif
  
  #define PDA_STACKOFFSET (5*8)
  
  #endif
  